Conventional optical switches based on micro-electro-mechanical (MEMS) actuated tiltable micro-mirrors in a linear micro-mirror array having a PIANO MEMS configuration, are detailed in U.S. Pat. No. 6,934,439 Mala et al, which is incorporated herein by reference.
An array of “Piano” MEMs mirror devices 21, 22 and 23, which pivot about a single axis of rotation θy above a substrate 25, is illustrated in FIGS. 1 and 2. Each mirror device 21, 22 and 23 includes a pivoting platform 26 defined by first and second substantially-rectangular planar supporting regions 27 and 28 joined by a relatively-thin substantially-rectangular brace 29 extending therebetween. Typically, each planar surface 27 and 28 is coated with a reflective coating, e.g. gold, for simultaneously reflecting a pair of sub-beams of light traveling along parallel paths, as will be hereinafter discussed. Each brace 29 acts like a lever and is pivotally mounted on C or I-shaped anchor posts 30 and 31 via first and second torsional hinges 32 and 33, respectively. The anchor posts 30 and 31 extend upwardly from the substrate 25. The ends of the first torsional hinge 32 are connected to the anchor post 30 and the brace 29 along the axis θy. Similarly, the ends of the second torsional hinge 33 are connected to the anchor post 31 and the brace 29 along the axis θy. Preferably, each of the first and second torsional hinges 32 and 33 comprises a serpentine hinge, which are considerably more robust than conventional torsional beam hinges. The serpentine hinge is effectively longer than a normal torsional hinge, which spans the same distance, thereby providing greater deflection and strength, without requiring the space that would be needed to extend a normal full-length torsional hinge.
A consequence of closely packed micro-mirrors is that the actuation of a single mirror will impart a torque, i.e. an angular rotation, onto adjacent mirrors as a result of fringing electric fields. In an effort to minimize this electrical cross-talk, electrode grounding shields 41, see FIG. 2, are positioned on the substrate 25 on either side of the first and second electrodes 36 forming electrode cavities, which are electrically isolated from each other. The electrode grounding shields 41 extend the length of the first electrodes 36, perpendicular to the axis of rotation θy of the platforms 26. The walls of the electrode grounding shields 41 extend upwardly above the upper plane of the first electrodes 36. The grounding shields 41 are kept at ground potential, i.e. the same as the mirrored platforms 26, while one of the first and second electrodes is held at an activation voltage, e.g. 100 Volts.
FIG. 3 illustrates an array of internal gimbal ring MEMs mirror devices 51 utilizing a first pair of serpentine torsional hinges 52a and 52b for pivoting a rectangular platform 53 about a first axis of rotation θy, and a second pair of serpentine torsional hinges 54a and 54b for rotating the platform 53 about a second perpendicular axis of rotation θx above a base substrate 55. The first pair of serpentine torsional hinges 52a and 52b extend from a single anchor post 56, which extends upwardly from the base substrate 55 through the center of the platform 53, i.e. at the intersection of the minor (lateral) and major (longitudinal) axes of the platform 53. Outer ends of the first pair of torsional serpentine hinges 52a and 52b are connected to a rectangular gimbal ring 58, which surrounds the first pair of serpentine hinges 52a and 52b, at points along the minor axes (θy) of the platform 53. The second pair of serpentine torsional hinges 54a and 54b extend from opposite sides of the gimbal ring 58 into contact with the platform 53, at points along the major axis (θx) of the platform 53.
The problem with conventional MEMS mirrors is the angular stability of the micro-mirrors in the micro-mirror array, wherein the angular position of the micro-mirrors drifts due to changes in the surface conductivity of the surrounding dielectric surfaces. The angular position of the micro-mirrors is also affected by the changing electric fields (cross-talk) in adjacent micro-mirrors of the array.
An object of the present invention is to overcome the shortcomings of the prior art by providing a bulk micro-machined ground plane with integrated cross-talk walls for use with electrostatic MEMS electrodes.